This invention relates generally to a fingerprint identification device to identify an individual fingerprint of a human.
The configuration of a conventional fingerprint identification device is shown in FIG. 1. A triangle prism 2 is positioned to contact a human finger 3. An optical beam from a optical source 4 such as a Light Emitting Diode (LED) is shot from the bottom of the prism. The optical beam is totally reflected by the inclined surface of the prism 2 and it goes through the focusing lens 5. The image of the finger 3 formed on the inclined surface of the prism 2 is focused on an imaging element 6 such as a CCD (charge-coupled device).
When the finger 3 touches the inclined surface of the prism in the finger identification device mentioned above, there will be a small space despite a tight contact between the finger and the inclined surface because of the convex and concave pattern of the fingerprint. The optical beam shot to the inclined surface of prism 2 will reflect totally at the small space where the concave portion of the finger 3 contacts the prism 2, and it forms a bright image on the imaging element 6. The optical beam will pierce the inclined surface at the tight contact where the convex portion of the finger 3 contacts the prism 2, and scatter on the finger surface. It will then form a dark image on the imaging element 6. Because of there features, a fingerprint pattern having bright and dark portions is obtained on the imaging element 6.
Fingerprint identification processing unit 7 will extract features of the fingerprint from the fingerprint image formed on the imaging element 6, and compare it with a database of fingerprints of many people previously stored in the memory in order to identify the specific fingerprint.
Conventional touch sensors, however, do not have a capacity to determine whether the object touching the prism is actually a human finger. While some of the conventional inductance, electrical conductivity and high-frequency radiation based sensors are not supposed to react unless they are touched by a human finger, it is relatively easy to operate the sensors by intentionally touching them with an object having characteristics similar to a finger. That is, the conventional sensors are unable to properly discriminate human fingers from other such objects. Detecting coils, for example, detect the presence of a human body before they are actually touched, so they have a problem of accuracy in that they cannot determine whether the sensor was in fact touched by the human body.
The object of this invention is to provide a fingerprint identification device which is able to detect the touch of a human finger with improved accuracy.
In one embodiment of the invention, a fingerprint identification device to identify if the input fingerprint matches with one of a plurality of fingerprints previously stored in a database includes a touch sensor for identifying a human finger, the touch sensor including 1) an oscillator unit which outputs a high-frequency signal; 2) an electrode unit which receives the high-frequency signal from the oscillator unit and which includes electrodes that are touched by the human finger to be detected; 3) a detector unit which outputs a signal which varies with the impedance of the electrode unit; 4) a discriminator unit which determines whether the item detected is alive; and 5) a reference signal setting unit in which a reference signal is stored prior to use to determine whether the item detected is a human finger.
In this touch sensor according to the present invention, the oscillator unit supplies a high-frequency signal to the electrode unit. If the item detected is a biological entity of a human body such as a finger, then the impedance of the electrode unit will change. The impedance is initially set to the impedance on the input side of the electrode when the finger is touching the electrodes. The reflected wave will be smaller due to the change in impedance when the finger is touching the electrodes. By detecting this reflected wave and comparing it with a reference signal in the discriminator unit, it can be determined that the reflection level is lower and that the electrodes are being touched by a person, but not an imitation finger.
The fingerprint identification unit according to this invention will determine that the input fingerprint is identical to the fingerprint in the database only when the touch sensor detects a human finger and the fingerprint processing unit detects the input fingerprint is identical to the fingerprint in the database. This feature will prevent cheating by using an imitation finger to ensure the accuracy of fingerprint detection.